This invention relates to a method of, and an apparatus for producing magnetic disks.
A method of producing a magnetic disk has heretofore been practised, as disclosed in U.S. Pat. No. 3,198,657 etc., in such a way that while rotating a disk, a magnetic coating composition having magnetic powder dispersed therein is dropped onto a first side (the upper surface) of the disk, that the rotational frequency of the disk is subsequently raised to centrifugally shake off the excess coating material and to form a coating, and that the coating is dried and thermally cured into a magnetic layer. In order to form a magnetic layer on a second side (the lower surface) of the disk with this method, it is necessary to turn over the disk and repeat the same procedure after the magnetic layer has been formed on the first side as described above. If the magnetic coating composition is applied onto the second side before the coating of the first side is fully dried, the coating of the first side becomes thinner due to the rotation of the disk in that case and the magnetic layer of desired thickness cannot be obtained. Even after the drying of the coating on the first side, if the second side is coated before the curing, the coating of the first side is adversely affected because the ambient atmosphere is filled with the vapor of a solvent in the coating composition attributed to the evaporation of the solvent. As stated above, therefore, a coating needs to be formed on the second side after the coating of the first side has been cured. As a result, the coating of the first side undergoes heating for the curing twice. Radiant heat to be absorbed during the curing of the coating becomes unequal, depending upon whether one side of the disk is a bare metal surface coated with nothing or is already formed with the coating. Therefore, a very slight difference in performance develops between the magnetic layers of both the sides of the magnetic disk manufactured by the above method.
Such difference is almost negligible when the thickness of the magnetic layer is great. However, the thickness of a magnetic layer in a magnetic disk rendered high in the recording density needs to be made approximately 1-3 .mu.m or less. In such case, the difference of the performances of both the sides poses various problems. Naturally, the job efficiency is inferior.
To the end of solving such problems or enhancing the job efficiency, it has been proposed and is disclosed in, for example, U.S. Pat. Nos. 3,730,760 and 4,033,288 that while holding a disk vertical or at an angle of 10.degree.-30.degree. with respect to the vertical plane, a coating material is applied onto both sides at the same time, whereupon it is dried and cured.
In forming coatings by the use of such method, there is involved the problem that when the excess coating material has been centrifugally shaken off, the coating material shaken off upwards falls onto the disk due to counter flow or counter dropping. To the end of preventing such splashing of the coating material, it has also been proposed that a casing for preventing the counter dropping is disposed near the outer edge of the disk so as to collect therein the coating composition shaken off.
However, the direction in which the coating material shaken off scatters varies to some extent due to complicated factors including the rotational frequency of the disk, the quantity of the excess coating material, the viscosity of the coating material, the temperature, etc. Even when, under the circumstances, one drop of splash strikes the inlet or some other place of the casing and then rebounds to fall onto the disk, the corresponding part of the disk becomes a defect. It is difficult to improve the structure of the casing so as to avoid such drawback in any case, and hence, the percentage of occurrence of non-conforming articles becomes high.